Clarify that some info defaulting to non-public may also default to being limited access within GitLab

While some non-public information will be shared with the entire GitLab team, types of information that we specify as always defaulting to non-public should also default to being shared on a need-to-know basis within GitLab, for similar reasons as to why it could not be shared publicly. some information that is not public may also need to be restricted to certain GitLab team members, teams, or roles.

We were already doing this in practice; for example, a lot of customer information is only available to certain parties within Salesforce and a lot of employee details are only available to certain parties within BambooHR. This clarifies the "not public" section in the Handbook to match.